The present invention relates to an improved PVO-zinc activated, lithium modified catalyst for use the in the partial oxidation of hydrocarbons to prepare dicarboxylic acids and anhydrides. More particularly, the invention relates to the improved phosphorus-vanadium mixed oxide catalyst prepared in an anhydrous system.
Basically, all of the methods used to prepare oxidation catalysts seek to obtain vanadium in a valence state of less than +5. One method of achieving this is to begin with vanadium in less than the +5 valence state. Another method and that used most widely in the art is to start with vanadium in the +5 state and reduce the valency to less than +5. This invention relates to the latter method. Several variations on this method have been used to obtain these catalyst. In one method V.sub.2 O.sub.5 is reduced in a solution with HCl to obtain vanadyl chloride. A typical catalyst preparation may involve dissolving the vanadium, phosphorus, and other components in a common solvent. The reduced vanadium with a valence of less than 5 is obtained by initially using a vanadium compound with a valence of plus 5 such as V.sub.2 O.sub.5 and thereafter reducing to the lower valence with, for example, hydrochloric acid during the catalyst preparation to form the vanadium oxysalt, vanadyl chloride, in situ. The vanadium compound is dissolved in a reducing solvent, such as hydrochloric acid, which solvent functions not only to form a solvent for the reaction, but also to reduce the valence of the vanadium compound to a valence of less than 5. Preferably, the vanadium compound is first dissolved in the solvent and thereafter the phosphorus and other components, if any, are added. The reaction to form the complex may be accelerated by the application of heat. The complex formed is then, without a precipitation step, deposited as a solution onto a carrier and dried. Generally, the average valence of the vanadium will be between about plus 2.5 and 4.6 at the time of deposition onto the carrier.
In another method the catalyst is prepared by precipitating the metal compounds, either with or without a carrier, from a colloidal dispersion of the ingredients in an inert liquid. In some instances the catalyst may be deposited as molten metal compounds onto a carrier. The catalysts have also been prepared by heating and mixing anhydrous forms of phosphorus acids with vanadium compounds and other components. In any of the methods of preparation, heat may be applied to accelerate the formation of the complex.
A method of obtaining vanadyl chloride was disclosed by Koppel et al, Zeit. anorg. Chem, 45, p. 346-351, 1905 by the reduction of V.sub.2 O.sub.5 in alcoholic HCl solution. This method has been recommended for the preparation of the phosphorus-vanadium oxidation catalyst for example, by Kerr in U.S. Pat. No. 3,255,211 where the solvent also serves as the reducing agent. Subsequently, U.S. Pat. Nos. 4,043,943, 4,251,390, 4,283,307; and 4,418,003 for example, employed this method generally referred to as the "anhydrous process" of reducing vanadium to prepare the basic phosphorus-vanadium catalyst. The catalysts produced by this latter method have been found to be generally superior to similar catalyst by the other methods. Specifically what had occurred to this class of oxidation catalysts prior to the return to the anhydrous process had been the addition of a veritable cornucopia of elements to the base vanadium-phosphorus composition, see for example U.S. Pat. No. 4,105,586 where in addition to V, P and O the catalyst must contain nine other elements. The catalyst were satisfactory, but manufacturing was difficult because of the number of components and their varying effects on the catalyst performance.
The anhydrous system went back to the basics with the Schneider procedure in U.S. Pat. No. 4,043,943 with only V, P and O. However, this catalyst required a very specific activation procedure as described, for example in U.S. Pat. No. 4,017,521. Barone (U.S. Pat. No. 4,251,390) showed that the addition of Zn alleviated the need for the specific activation process and produced a catalyst which was more easily activated and which was very stable to heat upset of the reaction system as well as exhibiting equal or superior performance (conversion/selectivity/yield) to the base catalyst. Small amounts of silicon and lithium compounds were also found to enhance the catalytic effects of P/V/Zn catalyst.
U.S. Pat. No. 4,147,661 discloses high surface area PVO mixed oxide catalyst additionally containing W, Sb, Ni and/or Mo at atonic ratios of 0.0025 to 1:1 to vanadium.
A particular problem facing all of the PVO containing catalysts is the loss of phosphorus, a discussion of this problem and various solutions is found in U.S. Pat. No. 4,515,899.
Many references disclosing oxidation catalysts which are suitable for producing maleic anhydride by the partial oxidation of n-butane, which catalysts contain molybdenum as one component of a phosphorus, vanadium mixed oxide catalyst. For example U.S. Pat. No. 3,980,585 discloses a catalyst containing P, V Cu and one of Te, Zr, Ni, Ce, W, Pd, Ag, Mn, Cr, Zn, Mo, Re, Sn, La, Hf Ta, Th, Ca, U or Sn; and U.S. Pat. No. 4,056,487 discloses a PVO catalyst containing Nb, Cu, Mo, Ni, Co and plus one or more of Ce, Nd, Ba, Hf, U, Ru, Re, Li or Mg. U.S. Pat. No. 4,515,904 discloses a procedure for preparing PVO catalysts which may include one metal of Mo, Zn, W, U, Sn, Bi, Ti, Zr, Ni, Cr or Co in atomic ratios of metal: V of 0.001 to 0.2:1.
U.S. Pat. No. 4,418,003 discloses PVO catalysts containing either Zn or Mo which is deactivated by Na or Li and which may also contain Zr, Ni, Ce, Cr, Mn, Ni and Al.
U.S. Pat. No. 4,251,390 discloses anhydrous process PVO oxidation catalyst activated with Zn and modified with Li or Si.
It is a feature of the present invention that the addition of a specific modifier to the phosphorus/vanadium/zinc/lithium mixed oxide catalyst produces catalysts of greater stability which give high yields of anhydride for long periods of time.
It is a further feature of the present catalyst that lower P/V ratios than unmodified catalysts are suitable with the concomitant reduction in the loss of phosphorus from the catalyst in operation.